The invention relates to a measuring device for measurement of parameters, in particular for measuring the temperature, in molten masses, in particular in molten metal or molten cryolite masses having a melting point above 500° C. The device comprises an optical fiber for receiving radiation from the molten mass and a cable reel having an external circumference for winding up the optical fiber and an internal space surrounded by the external circumference. The invention further relates to a cable reel suitable for use in the measuring device according to the invention.
Similar measuring devices are known, for example, from U.S. Pat. No. 6,227,702 B1 and German Patent DE 103 31 125 B3. Here, an optical fiber is used for measuring in molten masses, wherein the optical fiber is connected, on the one hand, to a measuring device and, on the other hand, to an immersion lance for immersion into the molten mass. The optical fiber is fed in through a feeding mechanism of the immersion lance and is guided, in a trailing manner, according to its consumption, wherein it is unwound from a type of cable reel. The measuring process is sufficiently well-known from the two patents cited above.
Regarding the relatively long optical fibers (up to several hundred meters) used, it has been evident that the fiber receives the radiation at its immersion end with a multitude of so-called modes, also called vibrational modes. The various modes are attenuated to different intensities along the extensive length of the fiber and are optionally modified by mutual interference. The process of attenuation, and thus of signal modification, is a function of the path length travelled by the light signal through the optical fiber. Overall, this may corrupt, for example, a temperature measuring signal of approx. 1,600° C. by distinctly more than 10° C. upon a change of the length of the optical fiber of approx. 100 meters, owing to it being consumed during the process. In order to keep the corruption effect as small as possible during multiple measurements, it is customary to keep a substantial length of optical fiber on the cable reel and not consume it, although this is expensive.
So-called mode filters, in which specific reduction of the diameter of an optical fiber in certain places eliminates modes, are known from the telecommunications industry. This can be done without difficulty in telecommunications, since the length of the fibers is relatively constant (the fibers are not being consumed) and the light is coupled into the so-called core of the optical fiber and not into its jacket. Like the core, the jacket consists of quartz glass. The length of the fibers used in telecommunications usually is distinctly more than one kilometer; modes in the jacket are deleted over such distances and have no interfering effect.
In metallurgy, in particular for measurements, substantially shorter optical fibers are used, usually less than 500 meters, wherein the light is coupled into both the core and into the jacket of the optical fiber and is very unstable as a result. The optical fiber being exposed to strongly varying temperature during its use can also impair the stability.